Electro-active materials are materials that deform or change their dimensions in response to applied electrical conditions or, vice versa, have electrical properties that change in response to applied mechanical forces. The best known and most used type of electro-active material is piezoelectric material, but other types of electro-active material include electrostrictive and piezoresistive material.
Many devices that make use of electro-active materials are known. The simplest piezoelectric device is a block of pre-poled, i.e., pre-oriented, piezoelectric material activated in an expansion-contraction mode by applying an activation voltage in direction of the poling.
Because piezoelectric devices are capacitive in nature, they exhibit a number of desirable mechanical and electrical characteristics. They have a very efficient coupling of energy from applied charge to mechanical strain, leading to a high bandwidth, a large force output and negligible resistive heating. Due to their capacitive nature, these devices draw their least current at zero rate of displacement. The stiffness of electro-active devices is determined by the electro-active material, which in general is crystalline, ceramic or polymer-based. However, as the electro-active effects are extremely small, e.g. in the order of 1 nm/V, the change in dimensions is relatively small and requires high voltages. Therefore, more complicated electro-active structures have been developed to achieve larger displacements.
To increase the displacements, several designs have been introduced such as stacks, unimorph or bimnorph benders, recurved benders, corrugated benders, spiral or helical designs.
For example, piezoelectric multilayer stacks can be fabricated by joining multiple piezoelectric rings or plates, such that the total displacement of the stack is the sum of the displacements of each individual plate. Inner electrodes separate adjacent plates. The stacks provide vertical displacement in accordance with their piezoelectric charge coefficients and the potential applied. Several hundred plates are necessary to provide total displacements of 10 or more micron.
A standard unimorph bender is made up of a flat piezoelectric strip bonded to a metallic shim from one side. Elongation of the strip when voltage is applied, forces the unimorph bender into a bent or curved shape. To increase the displacement range, the bimorph structures, mostly cantilevers-type structures, utilize two laminated piezoelectric layers, thus having two internal external electrodes to which voltages of opposite polarisation is applied. The application of an electric field across the two outer layers causes one layer to expand while the other contracts. This results in a bending motion with relatively wide displacements at the tip of the cantilever beam. In a cantilever configuration, the displacement of the tip is related to the length of the cantilever the applied voltage and the thickness of the cantilever. Cantilever-based piezoelectric actuators require lengths on the order of 25 mm or more to achieve a free deflection of 0.3 mm. It should be noted that a reinforced bimorph, i.e., a bimorph having a centre shim actually consists of nine layers: two piezo-ceramic layers, four electrode layers, two adhesive layers and the centre shim.
To further increase the maximum displacement of piezoelectric benders, it is known to form stacks and leveraged stacks, chains or extended sheets of benders. Such devices are described for example in the U.S. Pat. Nos. 3,816,774; 4,028,666; 5410,207 and 6,107,726. Stacked recurved actuator designs are described by J. D. Ervin and D. Brei in: “Recurve Piezoelectric-Strain-Ampliying Actuator Architecture”, IEEE/ASME Transactions on Mechatronics, Vol. 3, No.4, December 1998, 293–301.
Benders, stacks, tubes and other electro-active actuators are employed in a wide array of engineering systems, ranging from mnicro-positioning applications and acoustic wave processing to printing applications. Generally, actuators are used in such applications to generate force and effect displacement, for example, to move levers or other force transmitting devices, pistons or diaphragms, to accurately position components, or to enable similar system functions. Actuators employed for such functions typically are designed to provide a desired actuation displacement or stroke over which a desired force is delivered to a given load.
Depending upon design, electro-active actuators can generate a rotational or translational displacements or combinations of both movements.
Comparably large translation displacements have been recently achieved by using a helical structure of coiled piezoelectric tape. Such twice-coiled devices are found to easily exhibit displacement in the order of millimetres on an active length of the order of centimetres.
Whilst the piling of simple discs or blocks into stacks of electro-active material to achieve a cumulative change of dimension is a comparatively mature technique, it remains a very difficult task to join benders to stacks or to produce helices.
Therefore, it is an object of the present invention to provide novel configurations of electro-active material that—whilst maintaining similar performance than stacks of benders or twice-coiled benders—are easier to manufacture.